FATIGUE-STRENGTH OF TRIP-AIDED BAINITIC SHEET STEELS

Fatigue strength and crack initiation-propagation behavior of TRIP-aid ed bainitic sheer steels which are associated with the transformation- induced plasticity (TRIP) of retained austenite were investigated for the automotive applications. The steels composing of bainitic ferrite lath matrix and the retained austenite films completed the highest fat igue limit of several high-strength dual-phase type and bainitic type of steels. The threshold value of the stress intensity factor range (D elta K-th) was lower than those of the other dual-phase type of steels , although the crack propagation rare at a high stress intensity facto r range agreed well with those of the other steels. It was concluded t hat the retained austenite films suppressed a micro-crack initiation a nd propagation in the matrix and a long crack propagation due to ''str ess relaxation'' and ''block effect'' resulting from the strain-induce d transformation, because the mean interfilm space of retained austeni te is smaller than a plastic zone of crack tip.